The present invention relates to a sodium sulfur cell and more particularly to a safe economical method and means for filling the sodium compartment.
A conventional sodium sulfur cell includes separate containers for molten sodium and for molten sulfur. The latter container includes a main sodium compartment connected to a hollow solid electrolyte separator tube, conventionally formed of beta-alumina ceramic, with a closed end extending into the sulfur container. As illustrated in Christopher U.S. Pat. No. 3,740,206, the sodium fills the main sodium compartment and the separator tube. The sulfur container serves as the positive electrode, the sodium container as the negative electrode, and the separator tube as a solid electrolyte.
Filling of the highly reactive sodium in the sodium sulfur cell is a tedious process. In general, such sodium filling is performed under vacuum at temperatures above 100.degree. C. with molten sulfur present in the sulfur compartment. However, this is a hazardous procedure as the sodium is being filled in the vicinity of molten sulfur which react violently with it if direct contact occurs. Alternatively, the sodium can be added to the cell by electrolysis of sodium nitrate. However, this process is time consuming and expensive and so is not practical for commercial scale production of such cells. Vacuum filling of the sodium at temperatures above its melting point is the most common commercial method.
Another problem with sodium sulfur cells is that since sodium and sulfur are highly reactive in the molten stage, any break in the beta-alumina separator tube during operation which permits direct contact of these molten materials leads to very hazardous conditions. In that regard, further handling and transportion of the cells requires cooling of the sodium and sulfur from the molten state to the solid state. Such solidification and other dimensional changes during cooling stress the beta-alumina electrolyte tube as well as other seals in the cell which could cause cracking.